Vehicles having hybrid drives are being increasingly developed in which various drives are used for a drive task. The individual drives in the hybrid drive are able to cooperate in different ways. The individual drives either act together, partly together, or individually to operate the vehicle.
In so-called parallel hybrids, multiple drive units are coupleable to the drive shaft. Other hybrid designs are known besides the parallel hybrid vehicles, for example axle hybrid designs in which in each case at least one drive unit acts on one driven vehicle axle. A hybrid vehicle having two drive units is described in German Patent No. DE 10 2006 008 640. The vehicle has an internal combustion engine as a first drive unit, and has an electric machine as a second drive unit. The cited document discloses a method in which the internal combustion engine is started from the “solely electric driving” operating mode of the hybrid drive with the aid of a starterless direct start. It is also known that in certain configurations of the drive units (parallel hybrid, for example), the internal combustion engine that is present may be completely disconnected from the wheels and/or from the drive train with the aid of a separating clutch. It is thus possible to set the motor vehicle in motion, with the internal combustion engine not operating, solely via the electric drive (“electric driving”).
If, during electric driving, the driver requires a higher drive torque than the electric machine alone is able to provide, the internal combustion engine is started. For this internal combustion engine restart, the internal combustion engine may be set in motion via the slipping separating clutch. A portion of the drive torque of the electric machine is needed for starting and accelerating the internal combustion engine. To prevent the driver from withdrawing drive torque from the propulsion of the vehicle during this operation, the torque required for the restart is always kept as a “reserve,” so that for solely electric driving it is not possible to use the full torque of the electric machine for the propulsion of the vehicle.
Alternatively, it is possible in principle to set the internal combustion engine in motion via suitable fuel injections and ignitions, independently from the vehicle and vehicle drive. Such a start is referred to as a direct start. The direct start has been analyzed in conjunction with start-stop systems for nonhybrid drives. Under unfavorable conditions the direct start fails, so that for a nonhybrid vehicle, direct start is not suitable as the sole starting method. A failed direct start attempt generates considerable emissions, and therefore should be avoided whenever possible.